1. Field of the Invention
The present invention relates to a transformer including a detection winding arranged to detect an output voltage and to a transformer device including a transformer and a load circuit connected thereto.
2. Description of the Related Art
To apply a specific voltage to a load circuit connected downstream of a transformer, an output voltage of the transformer may be monitored to control the output voltage. One example of a monitoring method involves monitoring a detection voltage of a detection winding provided in the transformer in addition to input and output windings (see, for example, Japanese Examined Utility Model Registration Application Publication No. 6-9463).
FIGS. 1A and 1B are an illustration for describing a first configuration example of a traditional transformer; wherein FIG. 1A illustrates a partial cross-sectional view, and FIG. 1B illustrates a circuit diagram.
The transformer is made up of a roll 200 and a not-illustrated magnetic core. The roll 200 is made up of a tubular bobbin 204 and windings 201 to 203. The magnetic core is inserted in the tube of the bobbin 204. The bobbin 204 has a plurality of collars formed on its outer surface. The windings 201 to 203 are wound in winding regions between the collars (hereinafter referred to as sections). Specifically, the input winding 201 and the detection winding 203 are wound in a section adjacent to a first end, and the output winding 202 is wound in the other sections. The detection winding 203 is wound in a section different from the sections for the output winding 202 in order to isolate itself from the output winding 202.
In this transformer circuit configuration, the input winding 201 is connected between an input terminal 214 and a ground terminal 216. The input terminal 214 is connected to an AC voltage source. The detection winding 203 is connected to a voltage detector through a detection terminal 217. The output winding 202 is connected to a load circuit through an output terminal 215. For this transformer, a detection voltage proportional to an output voltage is detected by the voltage detector.
For the transformer having the above configuration, an input winding may be disposed at each of two sides of an output winding and the input windings may be connected in parallel in order to acquire strong connection between the output and input windings.
FIGS. 2A and 2B are illustrations for describing a second configuration example of a traditional transformer, wherein FIG. 2A illustrates a partial cross-sectional view, and FIG. 2B illustrates a circuit diagram.
The transformer is made up of a roll 300 and a not-illustrated magnetic core. The roll 300 is made up of a tubular bobbin 310 and windings 311 to 314. The magnetic core is inserted in the tube of the bobbin 310. The bobbin 310 has a plurality of collars formed on its outer surface. The windings 311 to 314 are wound in sections between the collars. The output winding 313 is wound in central sections, the first input winding 311 and the second input winding 312 are wound in sections adjacent to opposite ends, and the detection winding 314 is wound in the same section as that for the first input winding 311.
In this transformer circuit configuration, the first input winding 311 and the second input winding 312 are connected in parallel between an input terminal 321 and a ground terminal 322. The detection winding 314 is connected to a voltage detector through a detection terminal 323. The output winding 313 is connected to a load circuit through an output terminal 324. Also with this transformer, a voltage proportional to an output voltage according to the turns ratio between the output winding and the detection winding is detected by the voltage detector.
With the above transformer, for example, when the number of turns of the output winding is 1000, the number of turns of the detection winding is 10, and the output voltage is 1000 Vp-p, a detection voltage of 10 Vp-p is output to the detection winding.
For the above-described transformers, to acquire isolation, the output and input windings are spaced away from each other with the collar disposed between. Therefore, a leakage inductance between the both windings is large. Accordingly, if a capacitive load circuit that mainly has a capacitive component, such as a lamp or a photosensitive drum, is connected as the load circuit, the leakage inductance and the capacitive load circuit may be series resonant, depending on a condition, for example, such as a condition in which the frequency of an AC input voltage is close to a resonant frequency between the leakage inductance and the load capacity. If series resonance occurs, a leakage flux resulting from the leakage inductance increases.
A leakage flux is proportional to a series resonance current, and the series resonance current is proportional to a series resonance voltage occurring in a leakage inductance. The output voltage of the transformer increases by the amount corresponding to the series resonance voltage. Therefore, due to the series resonance, a resonance voltage proportional to the increase in the leakage flux occurs in the leakage inductance, and the output voltage of the transformer increases.
Due to series resonance, a detection voltage corresponding to a combined magnetic flux of a main magnetic flux and a leakage flux is output from a detection winding. FIGS. 3A and 3B are illustrations for describing a leakage flux occurring in a traditional transformer. FIG. 3A illustrates a transformer according to a first configuration example, and FIG. 3B illustrates a transformer according to a second configuration example.
For the transformer according to the first configuration example, a main magnetic flux 221 and a leakage flux 222 occur inside a magnetic core 220. The leakage flux 222 links the main magnetic flux 221 in the opposite direction at a linkage surface 223 of the detection winding. Accordingly, the main magnetic flux 221 and the leakage flux 222 cancel each other. During series resonance, the leakage flux 222 increases largely, so the main magnetic flux 221 is largely cancelled by the amount corresponding to the increase in the leakage flux 222, and the detection voltage reduces. Similarly, for the transformer according to the second configuration example, during series resonance, a main magnetic flux 321 is cancelled by the amount corresponding to an increase in a leakage flux 323 at a linkage surface 323, and the detection voltage reduces.
As described above, when an output voltage and a detection voltage are changed by the effects of series resonance, the accuracy of detecting an output voltage using a detection winding deteriorates.
FIGS. 4A and 4B are illustrations for describing changes in an output voltage and a detection voltage.
Here, results of experiments of applying an AC input voltage that has a constant magnitude with varying frequencies to a traditional transformer with an input winding-output winding-detection winding ratio of 1:180:1 and driving the transformer when a capacitive load circuit switches to 100 pF, 200 pF, or 300 pF are illustrated.
FIG. 4A illustrates the transformer according to the first configuration example. The output voltage of this transformer tended to increase with an increase in frequency. In contrast, the detection voltage of this transformer tended to reduce or remain virtually unchanged with an increase in frequency. Therefore, a calculated ratio between the detection voltage and the output voltage changed with respect to a change in frequency in a non-linear fashion.
FIG. 4B illustrates the transformer according to the second configuration example. In comparison with the transformer according to the first configuration example, the degree of each of the change in the output voltage and that in the detection voltage is smaller. However, similar to the transformer according to the first configuration example, the ratio between the detection voltage and the output voltage changed with respect to a change in frequency in a non-linear fashion.
As described above, for the traditional transformer, if the frequency varied, the accuracy of detecting the output voltage using the detection winding significantly deteriorated. This was more noticeable at larger capacitive values of the capacitive load circuit connected to the output winding.